Radiographic imaging

ABSTRACT

An X-ray imaging system includes a thin sodium iodide single crystal or matrix of single crystals for converting an X-ray image into a visible image. Preferably an electro-optical amplifier amplifies the detected image and delivers the amplified image to a closed circuit television viewing system.

States Patent [191 Paolini et a1.

RADIOGIRAPHKC HMAGHNG Inventors: Frank Paolini, Stamford, Conn.;

Alfred Kuhnel, Stow, Mass.

Assignee: American Science & Engineering, lnc., Cambridge, Mass.

Filed: Nov. 18, 1971 Appl. No.: 199,881

us. ca. 250/115 R, 250/213 R rm. C1. G0lt 39/18 Field 011 Search250/71.5 R, 213 R [56] References Cited UNITED STATES PATENTS 2,585,5512/1952 l-lofstadter 250/71 R 2902,604 v 9/1959 Baldwin 250/715 RDELIVERABLE ITEMS Primary Examiner-Harold A. Dixon Attorney, Agent, orFirm-Charles Hieken; Jerry Cohen [5 7] ABSTRACT An X-ray imaging systemincludes a thin sodium iodide single crystal or matrix of singlecrystals for converting an X-ray image into a visible image. Preferablyan electro-optical amplifier amplifies the detected image and deliversthe amplified image to a closed circuit television viewing system.

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RADIOGRAPHIC IMAGING BACKGROUND OF THE INVENTION The present inventionrelates in general to radiographic imaging and more particularlyconcerns novel apparatus and techniques for radiographic imagingcharacterized by low radiation levels, compact apparatus and directreal-time observation of the radiographic imagery.

Innovations in general medical radiology have led to typical decreasesin the dosage required for direct exposure of X-ray film and/or phosphorscreens of the order of several hundred. Photographic film emulsions areeffective photon detectors, but they require X-ray exposure levels muchabove that required for information transfer because a sufficient numberof emulsion grains must be activated to produce the picture densityrequired for direct observation.

Although electro-optical image intensification is known, the applicationof this technique to dental radiology has been limited by the moreconfined geometry of a dental examination. According to one prior artapproach a coherent fiberoptic bundle transmits the visual imageproduced at the tooth in a phosphor layer to a remoteimage-intensifier/vidicon camera. A phosphor is located in the imageplane, and a pulsed electron beam machine produces X-rays.

Still another prior art approach uses physically small yet highlyradioactive sources of radioactive Iodine- 125 in conjunction with X-rayfilm. A shortcoming of this film-based system is the relatively longtime required to expose X-ray film to an easily readable opticaldensity. Present source technology requires times of the order ofminutes for exposure while values of one second or less are considereddesirable so that subject motion does not significantly effect picturediagnostic quality. A system that solves some of the problems enumeratedabove also has utility in a package or human examining system searchingfor dangerous objects, such as guns, knives and bombs. Such a systemshould present no hazard to operating personnel, expose the package orperson being examined to negligible radiation doses and be reliable andcompact.

Accordingly, it is an important object of the invention to overcome oneor more of the problems enumerated above.

It is another object of the invention to provide a radiographic systemthat iss compact, sensitive to small doses of radiation and yet producesa useful image of hidden objects in real time.

It is another object of the invention to achieve one or more of thepreceding objects with a sensitive radiographic-energy-to-visible imagetransducer that is compact and relatively inexpensive.

It is a further object of the invention to provide a compact system inaccordance with one or more of the preceding objects suitable for use asa portable dental radiological system.

It is a further object of the invention to achieve one or more of thepreceding objects with a system that is relatively easy and inexpensiveto fabricate and capable of operating with a minimum of skilledattention.

SUMMARY OF THE INVENTION According to the invention, a radiographicsystem comprises a source of radiographic energy and means including athin sodium iodide crystal for converting the radiographic energy into avisible image after the radiographic energy from the source passesthrough an object. The source may be a low dosage isotope source, suchas radioactive iodine-125, conventional X-ray tube or other suitablesource. Preferably the system includes a closed circuit televisionsystem for amplifying and displaying the visible image of the detectorupon the face of a display tube for real time observation. The image mayalso be intensified and recorded on film.

According to a specific aspect of the invention especially useful forscanning parcels, there is means for scanning the object to be examined.Preferably this means comprises means for scanning the object to beexamined with the radioactive source and a number of contiguous ones ofsaid detectors, each with an associated photoelectric transducer forconverting the light signal from the associated sodium iodide detectorinto a corresponding electrical signal. In a preferred form of cullingstation according to the invention the output of the photo-electrictransducers is compared with a reference signal derived from a referencedetecting assembly to provide an indication when an unusual item issensed in the parcel being scanned.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. l is a block diagram illustratingthe logical arrangement of a system according to the invention;

FIG. 2 is a combined block-pictorial diagram of a portable filmrecording dental radiographic unit;

FIG. 3 is a combined pictorial-block diagram of a preferredsource-camera assembly for a dental radiographic system that isexceptionally compact and easy to use;

FIG. 4 is a more detailed view of the dental system source camera unit;

FIG. 5 is a graphical representation of the contrast between cavity andenamel images as a function of X-ray energy representative of requiredX-ray energy for a given contrast;

FIG. 6 is a graphical representation of X-ray transmission as a functionof energy;

FIG. 7 is a graphical representationof detection difference requirementsas a function of radioisotope source energy;

FIG. 8 illustrates the geometry of photons produced in the detectorcrystal;

FIG. 9 is a graphical representation of sodium iodide crystal detectorcharacteristics;

FIG. 10 is a pictorial representation of photon production as a functionof energy;

FIG. 11 is a pictorial representation of a parcel culling stationaccording to the invention;

FIG. 12 is a combined block-pictorial representation of an automaticparcel culling station according to the invention; and

FIG. 13 is a combined block-pictorial diagram of a parcel imagingstation according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to thedrawing and more particularly FIG. 1 thereof, there is shown a blockdiagram illustrating the logical arrangement of a system according tothe invention. A radiographic energy source assembly 11 emitsradiographic energy upon target 12 to produce an image on sodium iodidedetector 13 that is amplified by electro-optical amplifier 14 and thendisplayed on viewing system 15, such as a television picture tube.

The source assembly 11 typically comprises a small diameter source, ahousing, a shutter and a radiationshaping aperture. Source assembly 11functions to properly illuminate target 12 (a tooth or parcel forexample) and controls exposure for minimum operational dosage. Themaximum upper limit of dosage may be set by appropriately choosing theradioisotope and setting its initial activity level.

Sodium iodide detector 13 is a thin crystal that converts X- andgamma-radiation into a form of radiant energy that can beelectro-optically manipulated. This material is an exceptionallyefficient passive detector having the property of emitting violet lightwhen irradiated with gamma or X-rays. By making the crystal thin thecrystal reproduces spatial details contained in the probing X- orgamma-ray in violet light that could be visually observed unaided forsufficiently high X- or gamma-ray activity level.

However, to keep radiation levels low, the system preferably includesthe electro-optical amplifier 14 using known techniques to achieveelectron multiplication and input-output phosphor planes to intensifythe image. Simple optical lenses 16 guide the light from the detectorcrystal 13 to the input surface of the image intensifier 14.

Viewing system 15 comprises a television camera connected to a monitorthat may provide some light amplification to aid vision but primarilyfunctions to provide an easily viewed enlarged image at a locationconvenient to the analyst. Alternately, the image from image intensifier14 may be recorded on photographic film.

Referring to FIG. 2, there is shown a combined block-pictorial diagramof an exemplary system according to the invention. In FIG. 2 andthroughout the drawing corresponding elements are identified by the samereference numeral. Radiographic source assembly 11 includes a shutter 21that selectively emits radiographic energy that passes through target 12to reach sodium iodide detector crystal 13 to form an image that isfocused by lens 16 upon image intensifier 14. A radiation shield 20preferably encloses source assembly 11, target 12 and detector crystal13.

Viewing system 15 comprises a television camera 22, a camera controlunit 23 and a television monitor 24' intercoupled by the connectingcables.

Referring to FIG. 3, there is shown a preferred form of the inventionfor use as a portable dental radiographic unit. The source-camera unit24 includes the radiographic source assembly 1 1, the sodium iodidedetector crystal 13, the image intensifier 14, the lens 16 and thevidicon camera 22. All the elements of the system fit in the case 26.The system includes camera control 23, television monitor 24, powerconverter 27 for converting low battery voltages to higher operatingpotentials and image storage unit 31, which permits a radiographic imageto be stored after a short exposure and then viewed continuously ontelevision monitor 24. A system for storing radiographic images on astorage tube and then viewing it on a television monitor is known in theart and not a part of this invention.

Referring to FIG. 4, there is shown a more detailed view ofsource-camera unit 24 with portions in diametrical section to illustratecertain features of the invention. Radiographic source assembly 11includes a radioactive source 32 surrounded by lead rear and frontshields 33 and 34, respectively. Front shield 34 is formed with anaperture 35 that is covered by a protective shutter 36 except whenexposing a target. Sodium iodide detector crystal 13 is seated as shownat the entrance of a generally .I-shaped light pipe 37, preferably ofradiation shielding material, facing source 11 for positioningimmediately adjacent to target 12, in this case a tooth being X-rayed.

Light pipe 37 includes a lower mirror 41 and an upper mirror 42 at 45angles relative to the plane of sodium iodide detector crystal 13 toreflect the image on crystal 13 upon the input 43 of image intensifier14 through objective lens 44 and relay lens 45. With the constructionshown the source camera unit 24 may be of the order of only a foot longand less than 2 inches in diameter.

Having discussed the physical arrangement of a system according to theinvention, it is appropriate to consider the principles by which theinvention produces diagnostic radiographic images of quality comparableto those achieved with contemporary dental X-ray equipment.

The definition of image quality is highly subjective because it involvesthe ability of an observer to resolve fine detail and depends on thephysiological response of the eye and is subject to many variables inthe presentation of the image.

In evaluating system capabilities it is helpful to define thickness,density and absorption coefficients of damaged and sound toothstructures in addition to general performance criteria, such as theminimum spatial resolution and visually detectable contrasts. The sizeof the minimum resolvable tooth structure for diagnostic radiology isimportant because the required source strength is proportional to thenumber of resolution elements which make up the image field. Yet, thedimensions of the source are limited by the resolution requirement andthe exposure geometry. A preferred source balances these requirementsconsistent with the specific activity of suitable radio isotopes.

A convenient theoretical framework for such a study is described in anarticle by Carl 0. Henrickson entitled Iodineas a Radiation Source foeOdontological Roentgenology in ACTA RADIOLOGICA SUPP. 265. 1967). Thatarticle presents the pertinent physical parameters for the problem ofthe detection of a demineralized caries defect and defines the minimumacceptable contrast as a difference in the image density of the cavityand adjacent areas which is statistically significant to two standarddeviations. The resolution requirement can be estimated from theobservation that the panoramic images produced by that author inconjunction with others using a 0.5 mm diameter Iodine- 125 sourceplaced inside the mouth are of acceptable quality for diagnosticpurposes. That work is described in a paper by Beronius et al. entitledThe Use of Iodine-125 as an X-ray Source in Roentgen Diagnostics in 13INTERN, J. APPL. RADIAT. 253 (1962). In this case the source-filmdistance is only about three times the maximum object-film distance sothat the penumbra produced by the finite width of the source broadensthe image of each point in the highest object plane by 0.17 mm. Such aresolving power provided by the present system is adequate.

The cheek and tooth under examination attenuate X-rays provided by thesource for the detector. The detector crystal converts most ofthese'X-rays to visible light quanta. The fraction of the X-rays whichinteract in the detector crystal and the number of photons produced ineach such interaction along with the geometric efficiency of the opticalsystem at the photoelectron yield are relevant.

A basic limitation of the observable contrast with the present inventionis the statistical fluctuation in the number of photoelectrons producedin each resolution element at the photocathode of the image intensifier14. The remainder of the system may have gain sufficiently high tocontribute negligibly to statistical error. By determining the incidentflux of X-rays required to overcome the fluctuation and combining thatwith a number of resolution elements, the strength of the Iodine-125source to produce the desired image may be determined.

The Henrickson article cited above discusses the effects of the imagingdetector on the required flux of X- rays. While this discussioncontemplates X-ray film as the detection medium, it also presents thevalues of the minimum flux required to produce the desired differencefor the case of a perfect detector in which the contrast-limiting factoris the fluctuation in the arrival of the X-ray quanta. These minimumflux value determinations are applicable to the present invention bytaking into consideration the deviations of the present system from thecharacteristics of a perfect detector. An important feature of thepresent invention is that the sodium iodide crystal in combination withthe image intensification system according to the invention approachesthe ideal detector much more closely than film.

For an ideal detector the desired contrast may be defined in terms ofthe difference between the number of X-rays, N which emerge from thepath containing the cavity and the corresponding number, N for the pathcontaining just enamel. An acceptable contrast criteria is the number ofX-rays, N required to be incident on a l-millimeter section of the pathto produce a difference between N, and N, which is twice the estimatedstatistical fluctuation error. Referring to FIG. 5, there is shown agraphical representation of this difference as a function of X-rayenergy.

Referring to FIG. 6, there is shown a graphical representation of X-raytransmission as a function of energy through 1 millimeter of caries, 20millimeters of check, 1 millimeter of enamel and 5 millimeters ofenamel. The graphical representations of FIGS. 5 and 6 will be helpfulin analyzing a specific example.

Consider the situation at 27.4 keV. At this energy N, 22, the number ofX-rays required to be incident on the one mm section. From FIG. 6 (0.96X 22 21) are transmitted through the cavity while only 0.45 X 22 survivepassage through the enamel. The difference N, N, l l, and its estimatederror due to statistical fluctuation is V 10 2 5.5, so that thedifference is twice the estimated error.

For an ideal detector the number of X-rays which the source must emitinto each resolution element may be determined by dividing the valuesshown in FIG. 5 by the combined transmission of the cheek and the 5 mmof enamel, common to both paths, corresponding to the product of curvesB and D in FIG. 6. The results are graphically represented in FIG. 7 andare helpful in evaluating the source strength requirement for variousradio isotopes. This graphical representation shows a minimum number ofphotons per resolution element is required at about 40 keV energy.

Since the present invention does not use a perfect detector, the fluxlevels in FIG. 7 are increased by a factor related to the deviation ofthe system from perfection. Some of the X-rays pass completely throughthe detector crystal without interaction. The fraction which thuspenetrates depends upon the crystal thickness and the X-ray energy. Thecrystal thickness is preferably chosen at the largest value which willnot compromise resolution.

Referring to FIG. 8, there is shown the optical geometry at the detectorcrystal. The detector crystal 13 includes a specular reflecting surface51 upon which the X-rays are incident. The surface might also beabsorbing black to provide better resolution and half the number ofphotons. Each interaction produces a large number of visible photons,but only a small fraction of these are collected by the optics andfocused on the image intensifier photocathode 43. The transfer lens 16is focused upon the mirror surface 51 with the f3 viewing cone reducedto f5.3 within the sodium iodide because its index of refraction is1.77. The optical system accepts light quanta emitted into the internalf5.3 cones in either the forward or backward direction as shown for thehypothetical interaction in the center of the detector crystal in FIG.8. The specular surface 51 reflects the backward cone of light forward.

The preferred maximum thickness of crystal 51 may be calculated from theresolution requirement. The largest blur is produced by interactionswhich occur in the crystal at the maximum distance from the focal plane.Since the focal plane corresponds to that of the specular surface 51,this maximum distance occurs at point C in FIG. 8 with the resolutionelement width W being the thickness t of the crystal divided by the fnumber, 5.3. Thus, with a resolution requirement of 0.17 mm, the crystalmay be as thick as 0.9 mm.

Referring to FIG. 9, there is shown a graphical representation of thefraction of the X-rays which interact in the detector crystal forthicknesses of 0.9 and 0.45 millimeter as a function of X-ray energy.The sharp discontinuity at 33 keV is due tothe K-edge of the iodine inthe crystal. Note that interaction efficiency approaches unity for thethicker layer over most of the energy range. Thus, by dividing theoptimum fluxes shown in FIGS. 5 and 7 by the fraction of X-rays whichinteract, such as shown for sodium iodide of the indicated thicknessesin FIG. 9, and considering the statistical nature of the interaction inthe crystal, the statistical significance of the difference (N N may bemaintained.

Referring to FIG. 10, there is a graphical representation of photonsproduced per X-ray interaction in the detector crystal andphotoelectrons produced by the image intensifier photocathode, both as afunction of X-ray energy. The optical system collects approximately(21r/4) (t /41rf) where t is the crystal thickness and f is theeffective f number in the crystal, or about 4.5 X 10 of the light quantafrom each interaction for the crystal of FIG. 8 with a thickness of lmillimeter and for f/5.3.

The reflective losses at each of the optical surfaces are the order of10% and may be further reduced to negligible values by usingantireflection coatings. By using quartz lenses with negligibletransmission losses at the wavelengths of the scintillation,transmission losses through the lenses are negligible. With this opticalcollection efficiency and a 20% quantum efficiency of the imageintensifier photocathode 43 at the peak of the sodium iodidefluorescence spectrum (4,200A), an X-ray interaction which produces1,100 photons in the detector crystal will produce one photoelectron atthe photocathode 43 of the image intensifier. The number of electronsproduced per X-ray will be greater than one for all energies above 45keV and about 0.63 at 27.4 keV.

That the number of photoelectrons n thus produced is of the same orderas the number of X-rays (n) which interact in the detector crystalfacilitates estimation of the fluctuations of n. If n were much smallerthan n,

the percentage error would be 8n'/n' V; V lln If it' were much largerthan n, the statistics of n would dominate, and the percentage errorwould be approximately 1*!- In pretsnts e s whish thsuat q I. ri l" 1,the errors produced in n for an exact ratio P would be approximatelyequal to those which result from the fluctuations in P for a fixed(i.e., P6 nSP). The combined error may be estimated as Sn'ln' Vlln' llnVlinll l l on the assumption that the two distributions are largelyindependent.

The flux of X-rays N, required to be incident upon a 1-mm length ofcrystal to yield a difference accurate to more than two standarddeviations between the number of photoelectrons produced in the twoadjacent resolution elements may now be estimated. The number ofphotoelectrons produced are n, N,T eP and n, N,T eP, where T. and Trepresent the transmission of l millimeter lengths of enamel and cavity,respectively, and e is the fraction of the incident X-rays whichinteract in the detector crystal. The difference d eP,(T,.-T,) and itsestimated error is 8d= VeP(P+1) N,(T,+T The required two standarddeviation accuracy is achieved if d 28d, which occurs when N, (4/P) (P+1 T,+T,. T -Ty. At 27.4 keV, e 6.85,F (T631"Tg;6.96, T =0.45 and N, 62.This result is approximately 2.8 times as large as the value obtainedfor N, by Henrickson for the ideal detector plotted in FIG. 5.

There are other sources of noise; however, an increase of N, from 62 to68 to maintain the two standard deviation accuracy should be adequate.The effect of this additional noise is further lessened by reducing theexposure, the exposure being much less than one second for much of theuseful life of the radioactive source.

Turning now to estimating a suitable strength for an Iodine-125 source.The transmission of the cheek and 5 mm of enamel which X-rays musttypically tranverse to reach the last millimeter length of a typicaltooth is 8 X 10. The flux per resulution element incident on the cheekis therefore N, divided by this transmission, or 68/(8 X 10*) 8430. Withan intrinsic resolution of 0. 1 7 mm, the number of such elements per cmis approximately 3,500 so that net flux of X-rays entering the cheek percm is about 2.95 X 10. With the radioactive source at a distance of 5 cmfrom the specular surface plane 51, the fractional solid angle subtendedby this 1 cm area is 541m, or l/1001r. The total emission of the sourceinto 471' steradians must then be 2.95 X 10 X 314 9.3 X 10 X-rays. Ifthis is produced in 1 second, the equivalent source strength is 250millicuries, since one curie 3.7 X 10 disintegrations per second. Theself absorption of the 27.4 keV X-rays in the 0.5 mm diameter sourcesdescribed in the abovecited Henrickson publication is of the order of10-20% for densities of about 1 curie/mm This is more than offset by theaverage of 1.39 X-rays in the 27-28 keV range produced perdisintegration. Combining these two effects, a 0.5 mm diameter sourcecontaining approximately 215 millicuries of activity will provide thedesired equivalent source strength and hence produce the desired imagerywith a resolution of 0.17 mm.

An important feature of the invention is that this one second exposuretime is much less than that required when an Iodinesource is used withfilm as the detector. The exposures expected for film vary from 15-170seconds, with the longer times required for X- rays of thicker teeth,such as molars, in which the attenuation of the beam in the tooth itselfis greater. These values were obtained by correcting those reported inthe Henrickson article for various roentgenograms to a source strengthof 215 mCi and a source-film distance of 5cm. Thus, the 20-secondexposure reported for panoramic roentgenograms with a 300-mCi sourceinside the mouth at a distance of 2-4 cm from the film predictsexposures of 40-170 seconds in the present configuration. Similarly the10-20 second exposure range reported with a 500-mCi source at 6 cmdistance predicts values of 15-30 seconds in the present example withfilm substituted for the crystal detector.

Iodine-125 sources can be fabricated with up to 500 millicuries in the0.5 mm diameter package. The 0.5 mm size was selected by Beronius in theaboveidentified article to provide a maximum blur diameter of 0.17 mminpanoramic exposures taken with the source inside the mouth, where thedistances to the film vary between 2 and 4 cm. However, the 0.5 mmdiameter source is smaller than that which would produce such an imageblurring at a distance of 5 cm. A larger diameter source would betolerable if the distance is fixed at 5 cm as in the exemplaryembodiment. If the maximum tooth thickness is 6 mm as assumed herein,the distance between the outside surface of the tooth and the sourcewould be 43 mm. A 0.17 mm penumberal image of a point on the outsidesurface of the tooth would result from a source size of 0.17 mm X (43/6)1.2 mm. Under these conditions the effective area and activity of thesource may be increased by a factor of 5.8 without compromising theresolution. The maximum source strength would then be 2.9 curies, andthe exposure time would not be increased above 1 second until theactivity fell below 215 millicuries. This would require approximately3.75 half lives so that the useful life of the source would beapproximately 220 days.

If the exposure time can be extended to 2 seconds without causing unduehardship on the exposed part, the useful life of the source would extendover more than a year. Alternately, if the cheek can be pushed aside sothat the tooth can be irradiated directly, the required exposure timewould decrease by more than a factor of two so that the useful life ofthe source would be further extended.

Now consider the reduced radiation exposure resulting from the presentinvention. As stated above the equivalent activity of a 27.4 keV sourceof X-rays for producing the desired two standard deviation differencebetween adjacent healthy and cavity-containing regions of a tooth in a 1second exposure is 250 millicuries. With the source placed cm from thedetector crystal, it would be approximately 2.5 cm from the skin of thepatient. From FIG. 14 on page 34 of the abovecited Henrickson article al millicurie point source would produce a radiation field of 0.1milliroentgenslhour at a distance of 1 meter. Correcting this value forthe source strength and the 40-fold difference in source-skin distance,the source in the exemplary embodiment of this invention would producebut 0.1 X 250 X (40) /3,600 ll milliroentgens/second at the skin. Sincethe gram absorption coefficients of air and soft tissue are essentiallythe same at 27.4 keV, this converts to an equivalent skin exposure ofbut 9.6 millirads/sec., an exposure that is more than 100 times smallerthan the average mean exposure per dental film of l,l38 milliroentgensquoted in an article by Alcox entitled Diagnostic Radiation Exposuresand Dosages in Dentistry in 76 JADA 1066 (1968) as representative of thestate of the art in 1964.

Furthermore, that article explained that this exposure level onlydefines the properties of the radiation field and does not consider thesize of the area irradiated. Another advantage of the invention is thatthe source is so highly collimated that only the region of checkdirectly in front of the teeth being examined is irradiated. Theprotection of the eyes and other critical organs of the head-neck regionfrom radiation during dental X-ray has been a subject of recent concern.A low figure for a 14-film series of eye radiation is of the order of300-500 milliroentgens. With the shielding as disclosed in thisapplication the exposure to the eye will be below 1 milliroentgen,hundreds of times less. And while a conventional l4-film dental X-raysurvey under optimal conditions produces a dosage of between 0.5 and 1.0milliroentgen on the gonads, the system according to the inventionvirtually eliminates this exposure.

Table l is a list of the most commonly used radioisotopes which emit X-or gama-rays in the energy range of interest and their half-lives. Thefourth column lists the equivalent source strength at each energy forproducing the desired contrast in a system according to the invention.These values were obtained by scaling the results for Iodine-125 by thecurve shown in FIG. 7 for an ideal detector and correcting for thedetectorproduced increase in the fluctuations as described above. TableI follows this page.

For. energies below keV the absorption of radiation in the tooth itselfis so large that inordinately high activitiesare required to provide therequired number of X-ray quanta at the detector crystal. For energiesabove 100 keV the difference in absorption between healthy and diseasedtooth structures is so small that a very large flux is required toproduce statistically significant contrast between them. In addition theeffective utilization of these more energetic emissions is greatlyreduced because the conversion efficiency of the detector crystal fallsoff rapidly with increasing energy.

A preferred balance between the net transmission of the tooth and thedetection efficiency comes in the 35-50 keV range. Column 5 of Tablellists the approximate percentage of the nuclear disintegrations whichproduce the X-ray of interest in each case. It is seen that Iodine-125has the especially advantageous property of providing more than one 27.5keV X-ray per disintegration. The alternate sources may be used lesssatisfactorily for dental X-ray or with acceptable and perhaps superiorperformance for certain other applications.

The required isotope activity may be determined by dividing the desiredequivalent source strength by the radiative efficiency to produce theresults tabulated in the penultimate column of Table I. These resultsmay be compared with the last column listing the source activities whichare presently available commercially at reasonable cost. The comparisonshows that Iodinel25 is presently available at a source activity andsize compatible with dental X-ray in accordance with this aspect of theinvention.

Other suitable materials include Thulium-l and Americium-24l having anestremely long half life. As supplies of these materials become moreavailable and the size of the source package reduced, these materialsmight be advantageous when used in the dental X-ray aspects of theinvention.

Referring to FIG. 11, there is shown another embodiment of the inventionfor culling parcels. A gamma ray source assembly 61 scans a targetparcel 62 to produce an image of the contents along a line of detectorcrystals 63 backed by an array of photomultipliers 64 to producedetected signals analyzed by detector electronics 65, typically forproducing an alarm signal when the parcel contains a gunlike orbomb-like object. While not shown in FIG. ll, parcel 62 may be upon aconveyor that carries parcels across the culling station automatically.

It need hardly be doubted that parcels, baggage and people carryingguns, bombs or other contraband material create great dangers. Anddetection of such contraband material is difficult, costly and oftenembarrassing when reputable travelers are subjected to the indignitiesof a search. The present invention is capable of detecting firearmsunambiguously without opening parcels or subjecting them to abnormalphysical handling.

An important feature of a preferred system of FIG. 11 is its fullcompatibility with automated parcel handling systems. The inventionrelies upon distinguishing between the gamma-ray absorption propertiesof different materials to detect suspicious parcels. The imaging systemuses gamma-rays and electro-optical imaging techniques to identifycontraband within the culled parcels. The radiation doses produced bythe system present no hazard to personnel, and the parcels receivenegligible doses. The system may use electronic techniques well withinthe present state-ofthe-art with an assembly of commercially availablecomponents.

An inspection system according to the invention contemplates a two stageprocess in which the first stage is operated on-line to make a coarsedetermination of the amount of metal contained within a parcel (orperson in a personal examining system). This first stage does notrequire interruption of the transportation system of parcels or humansat all so long as the targets examined do not have a suspicious amountof metal.

Targets having a suspicious amount of metal move to a second stage whichdisplays an image of the target contents. For many applications, theimage may be manually examined to determine whether the target packageshould be opened or the target person searched. Where many fineexaminations must be made, it may be advantageous to have the secondstage include pattern recognition apparatus for automaticallydetermining whether the target contains contraband material.

In the first stage to be described in greater detail below, all parcelsare examined for dense material (especially metal) content, usingrelatively soft gamma-ray radiation of very low intensity withelectronic logical apparatus culling suspicious targets. Thesesuspicious targets then go to a second station, where the targetcontents are imaged upon a visual display for identification ofcontraband by an operator. The innocent targets may then return to themainstream while illegal targets are designated for other disposition.

The second station uses soft, low-intensity gammarays for obtaining animage of the contents. Radiation exposure is kept to only thatabsolutely necessary to present the information required for theparticular examination. And radiation exposure to the operator does notexceed the dose he receives daily from his natural environment. Afeature of the invention is that most radiation-sensitive products thatmay be shipped, such as very fast photographic film, are essentiallyunaffected by the radiation level required.

A complete parcel inspection system according to the invention maycomprise several metal detector culling stations operating in parallelin the main parcel stream before the sorting stations. A conveyor switchmay be arranged to immediately follow each detector section foractuation upon suspicious metal detection to divert the suspect parcelfrom the main parcel stream. A crossing conveyor may collect all suspectparcels and transport them to a single imaging station for detailedexamination. Parcels passing the image inspection may then return to themain parcel stream for further processing with the other legal" parcelswhile the illegal parcels may be switched to a storage area for furtherdisposition.

A feature of the invention is that the measurements obtained along withsuitable data processing reduces the false alarm rate, thereby reducingthe handling rate of the full imaging station Another feature of thegamma-ray detecting approach is insensitivity to magnetic effects. Incontrast a magnetic detector operates in an uncertain magneticenvironment. A sensitive magnetic detector must be located with carefulattention to proximity of electrical wiring and switch gear,characteristics of conveyor systems, proximity of machine and otherlocal ferrous metal traffic, such as carts and portable cleaningequipment. And weapons constructed of nonmagnetic materials would not bedetected by magnetic detection methods.

The culling station of FIG. 11 is compatible with conveyor speedscurrently in use in automatic and semiautomatic parcel handlingfacilities, these speeds typically being 200 feet/minute.

The gamma ray source assembly 61 may contain cobalt 57 radio isotope asthe radioactive source placed on one side of the conveyor belt. Detectorbank 63 may comprise a vertical array of 1" X l" gamma-ray sodium iodidedetector crystals, abutting one another. The source radiation beam ispreferably shaped to produce radiation fans that cross above the belt,beginning at the point source and terminating in the line of detectors63.

There are preferably at least two fan beams as shown to assure completeparcel coverage. Parcels passing through the two scanning beams modulatethe transmission to individual detectors in accordance with theabsorbing properties of the parcel and its contents. A suitablecriterion for defining a suspicious" parcel is to require counting ratesin one or more detectors to fall to less than a predetermined count. Asuitable minimum sized object of interest may be a cubic inch.

Referring to FIG. 12, there is shown a combined block-pictorial diagramof a preferred form of culling station for providing a rough indicationof suspect parcels. The scan beam control 71 scans a parcel to produce arough image of the transmissivity of the parcel upon the detectorcrystals 72 each backed by an associated one of photomuliplier tubeassembly 73 to provide signals that are processed and produce an alarmfrom alarm 74 when the indicated metal content is greater than apredetermined value.

An Hz oscillator 75 drives a motor 76 that rotates upper and lowerradioisotope sources 77 and 78, respectively, having their aperturesrelatively displaced by 180 for illuminating the parcel through upperand lower windows 81 and 82, respectively, in shielding case 83. Arotation sensor 84 provides a signal to the signal processing controlunit 85 to synchronize the data processing circuitry with thealternating scan beams.

The detection system comprises a number of detector crystals 72, theones designated 1 through 14, for receiving a rough image and areference detector crystal, designated R, for receiving referenceradiation. Each of these detector crystals is backed by the photocathodeof an associated photomultiplier tube and responds to incident radiationby providing light on the associated photocathode. A radiationconversion produces an electron emission that is amplified within theassociated photomultiplier tube. The output of each photomultiplier tubeis a series of electrical signal pulses occurring at a rate related tothe parcel material attenuation then in the field of view of thedetectors.

A light source 86 normally illuminates a photocell 87 that isdeenergized when a parcel passes between the light source 86 and thephotocell 87 to provide a signal delivered to signal processing controlunit 85 indicating that a parcel is present.

The signal processing apparatus comprises an associated one of counters91 and digital-to-analog converters 92. The channels associated withdetector crystals 1-14 also each include a sample and hold circuit andoutput switch 93 for sequentially delivering held potentialscorresponding to the count in the associated counter to the signal inputof comparator 94. The reference input of comparator 94 receives areference potential from threshold control 95 that receives thecombination of a manually set potential set by control 96 and areference potential from the reference detector channel representativeof the level of radiation then being provided from the scanning source.Signal processing control 85 provides appropriate signals for resettingthe counters, the sample and hold circuit, the output switches andactivating the threshold control 95 to p ovide a reference level.

The signal processing circuitry functionally counts pulses, converts thedigital count into an analog level and compares to establish whether ornot a detected pulse rate is to be accepted as an alarm. In theillustrated embodiment, the de tector Bear generates 14 parallel linesof variable pulse rate information. At a parcel passage rate of 200feet/ minute a complete signal processing cycle otiZS milliseconds 1sadeq ua t- The cycle is preferably divided into two parts to permitcount totalizing with respect to each scanned beam. By accumulatingcounts for 12.5 millesecond intervals, each count is representative ofthe parcel having traveled only 0.5 inch. Thus, in 1 inch of travel theparcel is examined from two different aspects with the detector crystalseffectively viewing the parcel through a 1 inch vertical slice that issegmented by the stacking of 14 individual inch-square crystal-windowedPM? assemblies. Thus, a high mass object as small as an inch square willeffectively shadow at least one detector to make the detector count solow that an alarm condition is signalled.

Having described the logical arrangement of the system of FIG. 12 andthe general functioning of the system, its operation will be describedin detail. As motor 76 rotates at constant speed, the beam from source77 exits through aperture 01 to illuminate detector crystals 72. Sensor04 provides a signal on input A of signal processing control unit 05signifying the start of the scan to produce a resetting signal thatresets the counters 91 to 0, thereby starting a new counter interval.Counters 91 then receive pulses from associated PMT assemblies 73 andpresent the advancing count to associated ones of digital-to-analogconverters 92. These converters provide an analog signal proportional tothe binary count. At the end of half the scan period (typically 12.5milliseconds) each of sample and hold circuits 93 receives a signal fromoutput D of signal processing control unit 85 that holds the potentialthereon to a value representative of the total count accumulated in acounter during the count interval.

As the scanning beam from source 77 is shielded, rotation sensor 84provides another signal to the A input of signal processing control unit85 that closes the switches in the sample and hold and switch circuits93 in sequence to compare the stored voltages in the differentialamplifier comparator 94 to provide an alarm when any of the storedpotentials is less than a predetermined reference potential provided bythreshold control 95. Alarm 74 may provide a signal that results inautomatic parcel switchout at the imaging station.

A feature of the invention is self-check cycling to introduce anappropriate threshold level under the control of the signal from outputline F of signal processing control unit 05 when no parcel is present.When light source 86 illuminates photocell 87 to indicate that no parcelis present on input line B of signal processing unit 05, channels l-Mcontinuously respond to radiation. If a predetermined rate is not thenproduced as determined from comparing the sampled voltages with areference potential determined by the output signal on output line F ofsignal processing control unit 85, comparator 94 produces an alarmsignal to indicate a malfunction. The time at which such an alarm signaloccurred would be indicative of which of the 14 channels was defective.

Another feature of the system is the provision of a reference detectorchannel to permit object signal count comparison against a floatingreference. Half life decay of the radiation source results in generalcount as being at cycles, may be adjusted to control the beam scan tocompensate for various conveyor system velocities.

Referring to FIG. 13, there is shown a combined block-pictorial diagramof an imaging station according to the invention for fine examination ofparcels rejected in the coarse examination effected with the system ofFIG. 12. A gamma ray source 101 illuminates parcel 102 to produce aradiographic image on scintillator detector 103 comprising sodium iodidecrystals to produce a visible image of the contents of parcel 102 thatis focused by optical system 104 on image intensifier 105 after whichimage detector 106, typically a vidicon camera, detects the intensifiedimage and delivers it to storage tube 107 through image control unit108. Image display unit 111, typically a television monitor, receivesthe stored image from storage tube 101 through storage control unit 112.An intensifier control unit 113 controls image intensifier 105.

The entire display is under the control of system control unit 114 whichdelivers appropriate control signals to intensifier control unit 113,image control unit and storage control unit 112. Such television displaysystems using storage tubes are well known in the art and not describedin detail here so as to avoid obscuring the principles of the invention.

Having described generally the physical arrangement of the imagingstation according to the invention, certain principles of operation willbe described. The gamma ray source 101 may be similar to that used atthe culling station, typically comprising cobalt-57 radioisotope. Thesource 101 provides a conical beam of gamma-rays that penetrate portionsof the parcel in varying degrees depending on the shape and mass of theinternal material. Rays which penetrate the package strike thescintillator detector 103 to produce visible light by photoelectric andCompton processees.

The scintillator detector 103 is preferably a mosaic assembly of smallcrystal planes of sodium iodide that produces a visible light picturerepresenting the contents of the parcel. The electrooptical systemamplifies this visible light picture so that it may be easily observedvisually.

Preferably the image detector 106 operates on a single frame basissimilar to taking a snapshot with a camera. A gamma-ray parcel exposureoccurs while the electron scan beam of the image detector 106 is cut offsufficiently long to build up a charge storage representing an image ofgood contrast and sufficient intensity for display purposes. Asatisfactory charge build-up or integration time interval isapproximately 1 second. Then the image detector 106 scans the targetimage charge with its electron scanning beam to produce a compositeelectrical video signal such as produced in an ordinary televisionsystem.

Although this video signal could be displayed as a visible imagedirectly on a standard television monitor picture tube, the single-frameraster-scanned image readout event is ordinarily too complex foroperator comprehension with one frame in l/30 second for standardtelevision. Therefore, an image storage tube electronically holds theimage for the length of time desired for repetitive scanning andsubsequent display and evaluation on the television monitor. Suitableoperator controls permit image erase at will to allow new exposures tobe made at various parcel positions.

The system described above minimizes radiation exposure requirements forsuccessful parcel content imaging, operates reliably and is relativelyeasy to assemble.

The resolution requirement for parcel contents imaging is not severe sothat many types of optical systems may be employed, such as diffracting,reflecting, Fresnel or combinations of these elements. The specificchoice of light transmitting characteristics of the optical systemaffects the source strength required and may be selected to maximizecost effectiveness.

A suitable source strength of 2.4 curie of Co-S 7 is adequate, althoughother sources may be used. For example, cadmium 109 may be used, thoughwith less conversion efficiency. A feature of the invention is the lowradiation from the system. The radiation near the operator at 1 meterdistance in an 8 hour working day with no shielding is of the order of0.1 milliroentgenens.

The daily dose acquired by a person from natural causes (cosmic rays andearth radioactivity) is larger and about one mr. The permissible AECdaily dose is 50 mr.

Each millimeter of lead shielding can further reduce the dose by afactor of 30, thus reducing the radiation received by the operator fromdirect parcel probing to insignificant levels.

Furthermore, the dose to the contents of a parcel is of the order ofseveral magnitudes below that required to change the density of veryfast film by 0.] density units, an effect that is directly unobservable.Such low levels of radiation make the system practical for examinationof a person.

Identification of contraband within a parcel or carried by a person maybe made by a human observer interpreting a television monitor image. itis within the principles of the invention to incorporate automaticidentification through the use of pattern recognition techniques.

There has been described novel radiographic imaging techniques andapparatus characterized by numerous advantages. The system is compact.Radiation doses are low. The invention is useful in solving manyproblems economically and safely.

It is evident that those skilled in the art may now make numerous usesand modifications of and departures from the specific embodimentsdescribed herein without departing from the inventive concepts.Consequently, the invention is to be construed as embracing each andevery novel feature and novel combination of features present in orpossessed by the apparatus and techniques herein disclosed and limitedsolely by the spirit and scope of the appended claims.

What is claimed is:

l. Radiographic apparatus comprising,

a source of radiographic energy,

means for directing said radiographic energy toward an object,

means including a thin crystal having a flat specular surface facingsaid source that is a compound from the group consisting of sodiumiodide and cesium iodide for converting at least the radiographic energypassing through said object into a visible image on the crystalrepresentative of the transmission characteristics of the object afterthe radiographic energy from the source passes through an object,

and lens means positioned with its object focal plane including saidflat specular surface for producing an image of said visible image,

said lens means including optical means for viewing the image on saidcrystal accepting rays within a predetermined viewing cone,

said apparatus characterized by a predetermined resolution W,

the thickness of said crystal being substantially equal to the productof said resolution W with the effective f number of said viewing cone insaid crystal.

2. Radiographic apparatus in accordance with claim 1 wherein saidthickness is less than a centimeter.

3. Radiographic apparatus in accordance with claim 1 wherein saidthickness is of the order of a millimeter.

4. Radiographic apparatus in accordance with claim 1 and furthercomprising,

a closed circuit television system for amplifying and displaying thevisible image on said crystal upon the face of a display tube for realtime observation.

5. Radiographic apparatus in accordance with claim 1 and furthercomprising,

an image intensifier for amplifying the visible image upon said crystal,and film means for recording the intensified image. 6. Radiographicapparatus in accordance with claim 1 wherein said source of radiographicenergy comprises a low dosage isotope.

7. Radiographic apparatus in accordance with claim 6 wherein said lowdosage isotope comprises an isotope from group consisting of radioactivelodine-l25 and Cobalt-57.

8. Radiographic apparatus in accordance with claim 6 and furthercomprising,

an assembly supporting said source and said crystal to define a regiontherebetween for accommodating a tooth to be X-rayed,

said assembly including optical means for focusing the visible image onsaid crystal upon a predetermined visible image plane.

9. Radiographic apparatus in accordance with claim 8 and furthercomprising image intensifying means located at said image plane.

10. Radiographic apparatus in accordance with claim 9 wherein saidoptical means comprises a generally J- shaped light pipe with saidcrystal at the end of the short leg of the J and the image intensifyingmeans at the end of the long leg of the J with said source supportedfrom said long leg.

2. Radiographic apparatus in accordance with claim 1 wherein saidthickness is less than a centimeter.
 3. Radiographic apparatus inaccordance with claim 1 wherein said thickness is of the order of amillimeter.
 4. Radiographic apparatus in accordance with claim 1 andfurther comprising, a closed circuit television system for amplifyingand displaying the visible image on said crystal upon the face of adisplay tube for real time observation.
 5. Radiographic apparatus inaccordance with claim 1 and further comprising, an image intensifier foramplifying the visible image upon said crystal, and film means forrecording the intensified image.
 6. Radiographic apparatus in accordancewith claim 1 wherein said source of radiographic energy comprises a lowdosage isotope.
 7. Radiographic apparatus in accordance with claim 6wherein said low dosage isotope comprises an isotope from groupconsisting of radioactive Iodine-125 and Cobalt-57.
 8. Radiographicapparatus in accordance with claim 6 and further comprising, an assemblysupporting said source and said crystal to define a region therebetweenfor accommodating a tooth to be X-rayed, said assembly including opticalmeans for focusing the visible image on said crystal upon apredetermined visible image plane.
 9. Radiographic apparatus inaccordance with claim 8 and further comprising image intensifying meanslocated at said image plane.
 10. Radiographic apparatus in accordancewith claim 9 wherein said optical means comprises a generally J-shapedlight pipe with said crystal at the end of the short leg of the J andthe image intensifying means at the end of the long leg of the J withsaid source supported from said long leg.